There are a number of apparatus which permit more-or-less pure oxygen to be breathed. The simplest of these devices employs a source of oxygen, and a method of delivering it to the lungs by a connection element such as a mask or mouth piece. The exhaled gas is exhausted into the air and thus lost for further use. These systems, called open circuit systems, do not conserve oxygen. This may be important since when breathing pure oxygen, only about 2.5% of the oxygen inspired is utilized by the body. The rest of the oxygen is exhaled with each breath and wasted. The exhaled gas from such systems contains about 97.5% oxygen. If this gas is allowed to accumulate in a closed room or chamber it may become a fire or explosion hazard.
It is possible to partly overcome the shortcomings of the once through systems by the use of a more complicated closed or semi-closed breathing system. In a semi-closed system part of the expired gas is retained in the system while the rest is exhausted from the system. The exhaled carbon dioxide is removed from the gas remaining in the system by a chemical carbon dioxide absorber, and the gas returned to the inhalation side of the breathing apparatus. Fresh gas is injected into the system to replace the volume eliminated with each breath. In contrast to the semi-closed system, a closed system retains all of the exhaled gas and passes it through the carbon dioxide absorber to remove the exhaled carbon dioxide.
While prior closed-circuit breathing apparatus provide for a more efficient use of the available oxygen supply than do the open-circuit or semi-closed systems, existing closed systems contain several weaknesses. Few, if any, closed systems use a tightly fitting, leak-proof connection element such as a mask. As a result, they do not deliver substantially pure oxygen to the subject. None of the closed systems can be purged efficiently and economically. In most instances, the present closed systems have certain dead spaces which do not permit the entire volume to be purged. Because of design limitations, even those without dead spaces require the use of an excessive amount of oxygen to completely exchange all of the gas in the system. The present rebreather units may lack a carbon dioxide scrubber which can be visually inspected while in operation to determine when the absorption granules have been exhausted and must be replaced.
In certain medical applications it is not necessary, or even desirable, for the subject to breathe substantially pure oxygen. There are other situations in which it is essential for the subject to breathe substantially pure oxygen, and that only substantially pure oxygen is being delivered to the subject. One such instance is when the subject is a diver being treated for decompression sickness. Another is when the subject is being treated in a hyperbaric chamber for such problems as carbon monoxide poisoning, gas gangrene, and many other diseases. During these treatments, it is important that the subject not be exposed to elevated levels of carbon dioxide.
One additional problem which must be recognized is that in some cases an injured individual must be evacuated from a remote area, and may be many hours away from a medical facility. In many such instances, the uninterrupted administration of substantially pure oxygen is the preferred treatment. Thus, it is important that the breathing apparatus be able to deliver substantially pure oxygen at all times, and that it use oxygen as efficiently as possible. The breathing apparatus must be rugged, reliable, and simply constructed since it will often be used under primitive field conditions.
It should be noted that the use of pure oxygen is important and even essential to life in many types of medical problems. Some of these needs are found in diving or caisson accidents, heart attacks, asphyxiation, gas gangrene, carbon monoxide poisoning, near drowning, smoke inhalation, and many others. Most, of the prior breathing devices including those described in U.S. Pat. Nos. 575,167; 4,163,488 and 3,929,127 have tried to meet various of these needs. However, all of them fail to solve some of the essential requirements, such as the need for very efficient use of the available sources of oxygen, and a completely reliable delivery of substantially pure oxygen without the presence of carbon dioxide.